Cleaning up the Bath water

Bath's elegant image has been sullied by the efflux from combined sewer overflows. But not for much longer, discovers Andrew Mylius.

Sunlight glints on the ripples spreading out behind a couple of rowing boats and a canoe gliding up the River Avon. The air is fragrant with newly cut hay, warm earth and lush vegetation. Fishermen watch their floats like herons, there are couples romantically wandering the riverbanks, and the squawk and squeal of playing children intermingles distantly with birdsong.

The Elysian moment is shattered, though. Dangling grossly from the branches of nearby trees, condoms, tampons and swags of tatty paper record the river's last high water line.

For the tourist who is rounding off a visit to Bath by exploring the Avon valley on foot, by bike or in a boat, and for the hundreds of leisure-seeking Bathonians who follow the river on its picturesque path towards Bristol, the detritus-draped bushes taint an otherwise idyllic setting.

To make the lower reaches of the River Avon a fitting complement to a city famed for its buttercoloured Georgian architecture, hot springs, Roman history and Jane Austen novels, Wessex Water has ordered a £28M project to cut discharges from nine combined sewer overflows (CSOs).

Water and sewerage authority Wessex has already completed remedial work on over 100 CSOs in central and western Bath. This has delivered some improvement in water quality but, more significantly, has cut the amount of flotsam and jetsom flushed from the city's foul sewers and into the river each time there is heavy rain.

'River water quality is not the primary issue, ' says Drummond Modley, manager of an alliance between Wessex Water and consultant Montgomery Watson Harza which is acting as client for the Bath CSO project. 'The Avon is a large, healthy river.

Water quality is hardly compromised by the discharge of sewer waters. This scheme is primarily about aesthetics.'

MWH Wessex has appointed a joint venture of consultant Carl Bro and contractor Costain to design and build a new system of storm water storage tanks and a second rising main to manage some 300,000m 3of combined foul and storm water that would otherwise end up in the river.

Bath's sewer system is Victorian.

Modernising it by separating rainwater drainage from the foul sewer would involve work on a vast scale, bringing huge disruption to the city's crowded streets.

It would be 'pretty unpopular', Modley understates.

A 700mm cast iron rising main carries sewage the 8km from Wessex Water's Midland Road pumping station in the light industrial, commercial and residential western reaches of Bath to treatment works in the village of Saltford. It handles peak flows of 520 litres/second. A new 700mm diameter, ductile steel rising main will double the system's carrying capacity. But this alone is not enough to halt major discharges, and three storm tanks are to be built to provide large scale additional holding capacity.

The new scheme will start east of the existing pumping station. A 1.5m diameter microtunnelled pipeline is to be installed from Norfolk Crescent - a little known Georgian beauty bordering on an area earmarked for major urban regeneration - to carry sewage and storm water under gravity from Bath's northwestern catchment to the Midland Road pumping depot. Here, the existing 10.5m diameter, 18m deep wet well provides an initial buffer against storm waterinduced flow surges.

Existing storm tanks at Midland Road will be decommissioned once the new rising main and giant underground tanks are completed. These will be real bunkers - 15m in diameter by 18m deep at Windsor Villas, and 10.5m diameter by 18m deep at the Lower Bristol Road and the Environment Agency's Bath depot sites. Sewage will be pumped from the tanks into either the old or the new rising mains once the storm flows have abated. Three new 40m diameter, 7m high concrete tanks are also to be built at the Saltford treatment works.

Planning permission is hardwon in Bath, a world heritage site flanked on all sides by conservation areas and nestling in an area of outstanding natural beauty. Wessex Water applies for planning consent next month and has been at pains to minimise visual impact, risk and disruption to the city in developing the scheme.

Bath's hot springs have had a major influence on the choice of design solution, as it is feared that disturbing the underlying geology could irrevocably damage flow to the spring. Major tunnelling was ruled out as too risky in the highly fractured rock close to the city centre.

Extensive site investigation has been undertaken and the relatively short stretch of microtunnel is shallow. But exhaustive borehole testing is being carried out at the storm water tank sites where excavation will be carried out using drill and blast techniques. Construction, which should start early next year, will be closely monitored by Bath & North East Somerset Council and by specialists in Bath's geology from Imperial College London, says Modley.

Tanks and pipeline will be invisible. Apart from 600m of rising main that will have to be installed in the road as it leaves Bath, it will be unobtrusively tucked away in a trench to one side of the old Bristol-Bath railway line which runs north of the river and is now used as a cycle path. Directional drilling will be used to take the main across the river west of Midland Road and, again, as it enters the Saltford conservation area.

Meanwhile, to keep sewage stored in the tanks 'sweet' - that is, to prevent it going septic and smelling - it will be chemically aerated with calcium nitrate, also known as Nutriox. And the tanks will be automatically washed out with water jets as soon as they are empty to prevent build up of malodorous solids.

Overflow will not be entirely eliminated, Modley confesses.

The CSOs will still discharge into the Avon three or four times a year. But compared with the dozen or more spills that take place now it will be a vast improvement - and what the new system cannot store and treat will be screened.

Storm surge study

Water companies across the UK are tackling discharges from close to 8,000 of the 25,000 combined sewer overflows in England and Wales under the current AMP3 spending round. Sudden surges in flow caused by storms often exceed sewer capacity, and are discharged into nearby watercourses. In doing so solids that have settled in the sewer's invert are flushed out.

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